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Wearable medical devices making use of flexible
thin film sensors fabricated using techniques
such as inkjet printing, electroplating,
lithography, etc. are attracting a lot of attention.
These devices have advantages of being light
weight as well as conformable to complex
surfaces. However, they need to be interfaced
with rigid electronic devices for data processing
and communication. These Flexible Hybrid
Electronic (FHE) devices have the advantages of
flexible electronics as well as the performance of
conventional rigid electronics [1]. While FHE
provide an enticing prospect, they also have
their own challenges as far as reliability is
concerned.
This work reports reliability aspect of a project
aimed at designing and fabricating a Wearable
Sensor Patch (WSP) used to monitor
electrocardiography (ECG) signals. In Phase I of
our project, the WSP was fabricated with rigid
electronic components mounted on one side
(component side) of a flexible Kapton® polyimide
substrate, whereas the ECG electrodes were
printed on the other side (sensor side) and
connected to the electronic components using
Through Hole Vias (THVs). The polyimide
thickness was 2 mil and Cu trace thickness was
2 µm. SnPb solder (reflow temperature: 204oC)
was used to connect electronic components to
Cu traces. The front-end analog to digital chip of
this device was susceptible to failure due to
cracking of flexible Cu traces close to solder
joints, reducing robustness of the devices in real
life use [2]. This issue was addressed in phase II
of the project where the effects of Cu trace and
Kapton® polyimide thickness and the use of low
reflow temperature SnBi solder (reflow
temperature: 175oC) on device reliability were
investigated. Cu traces of 2 and 6 µm thickness
and Kapton® polyimide substrate of 2 and 5 mil
were used. SnBi and SnPb solders were also
compared. Devices with different combinations
of Cu trace and polyimide thicknesses and using
either SnBi and SnPb solder were fabricated and
flex tested to find out which combination was
most robust. The Cu trace geometry was also
modified.
Studies in the past have investigated behavior of
flexible circuits under cyclic flex testing including
flex testing of circuits fabricated using inkjet
printing, physical vapor deposition as well as
electroplating [3], [4], [5]. However, this
behavior is not necessarily replicated in an actual
device as circuits have to go through thermal
cycling during solder reflow process, inducing
stresses due to CTE mismatch. Hence flex
testing the final device itself is necessary. Joint
locations of the front-end analog to digital chip
were first examined using a Zeiss light
microscope and imaged to get a baseline set of
images. Any defects during manufacturing
process were also documented during the
imaging process. Imaging was done in reflection
mode as well as transmission mode (against
bright backlight), and with both component side
as well as sensor side facing upwards. Any
visible light coming through the Cu traces in the
backlight mode indicated complete failure at that
location. Each device was then bend tested
using a 4” radius of curvature mandrel for 1000
cycles, with the mandrel pushing against the
sensor side. The devices were examined using
the microscope for new damage due to the bend
testing by comparing to the baseline images and
any new damage observed was imaged at the
same magnification. The same process was
repeated with 3”, 2” and 1” radius of curvature
mandrels to simulate mounting of the WSP on
various locations of human body. The process
was also repeated with 2” and 1” radius of
curvature mandrels pushing against the
component side to simulate peeling off of the
WSP from human test subject. Effect of
improved Cu trace and tab design with wider
traces and rounded corners was also studied
using a similar bend testing protocol.
It was observed that only devices with 6 µm
thick Cu traces, SnBi solder and 2 mil thick
Kapton® polyimide had no defects after the
fabrication process. One of the reasons might be
use of lower reflow temperature solder which
results in lower residual stresses after thermal
cycling. Devices with all other combinations had
a few defects as a result of thermal cycling they
went through during the fabrication process. It
was also observed that the same devices that
had no defects after fabrication also performed
best during flex testing with fewer new defects
being observed as a result of flex testing. Higher
robustness against bending due to increased
thickness might be one of the reasons. Complete
failure was observed more in assemblies using 2
µm thick Cu traces. Improvement in Cu trace and
tab design also helped to avoid complete failure
and none of the assemblies that had the
improved design failed completely.

Varun Soman, Graduate Research Assistant
Binghamton University
Binghamton, New York
USA